1. Technical Field
The invention relates to a coaxial high-frequency cable which has an inner conductor, a dielectric spacer surrounding the inner conductor, and a tubular outer conductor which is applied over the dielectric spacer and is concentric with the inner conductor.
2. Description of the Prior Art
Coaxial highxe2x80x94frequency cablesxe2x80x94hereinafter referred to as xe2x80x9cHF cablesxe2x80x9dxe2x80x94are used, for example, as interconnecting cables between a transmitter and an antenna for transporting electromagnetic waves. Such cables can also be installed, for example, in tunnel sections as a xe2x80x9cradiatingxe2x80x9d HF cable. In conventional embodiments, the dielectric disposed between the inner conductor and the outer conductor functions as a spacer so as to hold the two conductors coaxially or concentrically and at a fixed separation relative to each other. The dielectric can be in the form of a tightly packed dielectric made from an insulating material which completely fills the space between the inner conductor and the outer conductor, either in solid form or in form of a foam. The dielectric can also be a so-called hollow dielectric with spacers made of an insulating material which contact both conductors. Such spacers can, for example, be in the form of a helix, in the form of axially spaced-apart disks and also in the form of individual elements. The HF cable is operational as long as the inner conductor and the outer conductor are undamaged and positioned concentrically relative to each other. The insulating material of the dielectric, however, can melt at very high temperatures, in particular in the event of a fire. The inner conductor and the outer conductor will come into contact within a very short time, thereby rendering the HF cable useless.
The spacer of the HF cable which is described in EP 0 503 129 A1 referenced above, is constructed in the form of a profiled helix or made from individually formed pieces of glass or ceramic. When these materials are employed, the corresponding HF cable can also be used at higher temperatures. The cable will also operate in the event of a fire, at least for a limited time. However, because the materials used for the spacers are quite brittle, the HF cable is rather expensive to manufacture. These materials also degrade the electrical properties of the cable.
It is the object of the invention to construct the HF cable of the type described above so that the cable can be manufactured with existing technology while simultaneously improving its electrical properties.
The object is solved with the invention in that the spacers are made from a combination of a thermoplastic insulating material and a mineral material.
The insulating material employed for this HF cable comprises two different components. When a thermoplastic insulating material is xe2x80x9cfilledxe2x80x9d, for example, with mineral particles, the HF cable can be manufactured with conventional processes, wherein the spacers in particular can have any shape and form. Conventional manufacturing techniques can also be employed when disks made of thermoplastic insulating material are used in conjunction with disks made of mineral material or other suitable individual support members. The thermoplastic insulating material is also an effective spacer when the HF cable is transported and installed. Moreover, the two-component material has significantly better dielectric properties than a purely mineral material, which helps to improve the transmission characteristics of the HF cable. When the mineral material is used, the inner conductor and the outer conductor do not contact each other at high temperatures or more particularly, in the event of fire, even after the thermoplastic material has melted. The HF cable therefore remains operational for a considerable period of time. When the two-component material is used for the spacers, the HF cable is easy to manufacture, has excellent electrical properties and operates even at high temperatures.
The invention will be fully understood when reference is made to the following detailed description taken in conjunction with the accompanying drawings.